**Core Concept**
The question is inquiring about a biological marker associated with antisocial behavior in boys, which suggests a link between neurobiology and psychiatric disorders. This is a topic of interest in the field of psychiatry, particularly in understanding the underlying causes of antisocial behavior.

**Why the Correct Answer is Right**
Research has identified a link between low levels of the hormone oxytocin and an increased risk of developing antisocial behavior in boys. Oxytocin is often referred to as the "cuddle hormone" due to its role in social bonding and attachment. Low levels of oxytocin have been associated with difficulties in forming and maintaining relationships, leading to increased aggression and antisocial behavior. The oxytocin system is influenced by genetic factors, which may contribute to the development of antisocial behavior in boys.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect as there is no established link between testosterone levels and antisocial behavior in boys. While testosterone does play a role in aggression, the relationship between testosterone and antisocial behavior is complex and not as straightforward as the link between oxytocin and antisocial behavior.

**Option B:**
This option is incorrect as there is no established biological marker for antisocial behavior in boys. While certain genetic markers may be associated with an increased risk of developing antisocial behavior, there is no single biological marker that can reliably predict antisocial behavior in boys.

**Option C:**
This option is incorrect as there is no established link between serotonin levels and antisocial behavior in boys. While serotonin does play a role in mood regulation, the relationship between serotonin and antisocial behavior is complex and not as straightforward as the link between oxytocin and antisocial behavior.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that antisocial behavior in boys is a complex issue, influenced by a combination of genetic, environmental, and social factors. A comprehensive approach, including early intervention and treatment, is necessary to address the underlying causes of antisocial behavior.

**Correct Answer: D. Oxytocin**